Silicones have a special position among elastomers, since their mechanical parameters and their physical properties change only slightly with temperature. In a unique fashion, extreme cold flexibility is combined with excellent hot-air resistance. However, at high temperatures of above 180.degree. C., even silicones are subject to appreciable heat ageing which in the case of silicone elastomers leads to changes in the mechanical properties such as, Shore hardness, tensile strength and elongation at break. Possible causes are inter alia, oxidative processes, thermal chain scission and also reorganization of the siloxane network under the catalytic action of inherent impurities, often in association with unavoidable traces of water. Furthermore, dimethylsiloxane chains having terminal siloxanol groups can be thermally degraded with elimination of low-molecular-weight rings.
The hot air ageing of silicone vulcanizates generally becomes apparent purely qualitatively in an embrittlement of the material which increases with temperature and time and is associated with an increasing loss of elasticity.
The thermal and oxidative degradation of silicones and their stabilization by suitable additives has been studied for some time, with transition metals and their compounds having had particular importance from the beginning. On this subject, reference may be made to J. M. Nielsen: "Oxidative Stabilization of Silicone Fluids" in: "Advance in Chemistry Series 85 (Stabilization of Polymers and Stabilizer Processes)", Ed. R. F. Gould, American Chemical Society, Washington D.C. 1968. As heat stabilizers for silicone rubber, use is most frequently made of compounds of the lanthanides, in particular cerium, but also compounds of iron, zirconium and titanium, preferably the 2-ethylhexanoates, chlorides, oxides and siloxanolates.
Copper compounds as heat stabilizers for silicone elastomers such as bis(ethylenediamine)copper(II) sulfate as described in U.S. Pat. No. 4,777,087 (Xerox Corp., issued on Oct. 11, 1988) or copper salts of carboxylic acids, such as copper(II) 2-ethylhexanoate, as described in U.S. Pat. No. 2,999,076 (Dow Corning Corp., issued on Sep. 5, 1961), have already been disclosed. In these publications, from 1 to 20 parts of the copper ethylenediamine complex and from 0.001 to 0.40 parts of copper as carboxylate (corresponding to from 10 to 4,000 ppm of Cu) respectively are used per 100 parts of organopolysiloxane. DE-C 27 52 097 (Sustova et al., issued on Apr. 1, 1982) describes a process for stabilizing polymers against thermal-oxidative degradation by mixing the polymers with a formate of a metal of changing valence, e.g., copper, with the salts being added in amounts of from 0.1 to 20 parts by weight per 100 parts by weight of polymer. Reference is again made to less than 0.1 parts by weight of salt per 100 parts by weight of polymer leading to an insignificant stabilizing effect.
U.S. Pat. No. 3,435,000 (General Electric Co., issued on Mar. 25, 1969) and U.S. Pat. No. 5,086,107 (Shin-Etsu Chemical Co., issued on Feb. 4, 1992) describes the use of zinc as carbonate or hydroxide for improving the oil resistance of silicone elastomers.
2-Ethylhexanoic acid is usually a constituent, as a counter ion, of heat stabilizing transition metal compounds, but has also been used in addition to acetylacetonates of iron, nickel and cerium, but not of copper. On this subject, reference is made to U.S. Pat. No. 4,528,313 (Dow Corning Corp., issued on Jul. 9, 1985).
Copper and zinc as heat stabilizers are disclosed in U.S. Pat. No. 4,824,903 (Dow Corning K.K., issued on Apr. 25, 1989). They are claimed individually or together in the form of their oxides, phosphates, silicates, carbonates and sulfates in amounts of from 10% to 90% together with acrylated silicone resins as constituents of insulation coatings of electric metal sheets. Reference is made to DE-B 26 07 185 (Nippon Steel Corp., issued on Dec. 22, 1977). Copper undecylenate and zinc acetylacetonate, are also used as heat stabilizers for UV-curable acrylamidepolysiloxane compositions in amounts of 1% by weight. Reference is made to, U.S. Pat. No. 5,246,979 (Dow Corning Corp., issued on Sep. 21, 1993) or the corresponding EP 518 142 A1.
DE-B 25 43 849 (M. P. Grinblat et al., issued on Apr. 14, 1977) describes the use of copper silicate, sulfide, borate or phosphide in an amount of from 0.2 to 50 parts by weight per 100 parts by weight of organopolysiloxane for thermally stabilizing (HTV-)organopolysiloxane compositions. The copper salts specified are used in combination with metal oxides, for example, from 2 to 10 parts by weight of zinc oxide per 100 parts by weight of silicone rubber, for improving the resistance of the vulcanizates to degradation under conditions of limited air access, reference being made to less than 0.2 part by weight of copper salt and 2 parts by weight of zinc oxide not leading to the desired improvements.